Shape memory actuators produce displacement or force by forcing a suitable actuator element to undergo a transition between a low and a high temperature phase (martensite and austenite, respectively), each phase having characteristic dimensions. The necessary energy is most commonly supplied by ohmic heating and removed by any convenient mechanism, such as conductive, convective or radiative heat transfer. The use of SMA-based actuators is steadily growing owing in large part to their propensity for being conveniently packaged into narrow form factors. This ability is related to the fact that shape memory alloys are shaped into wires of modest cross section and sufficiently long length to achieve useful stroke during their activation. The slender shape is what permits insertion of SMA wires into narrow spaces.
As shown in FIG. 1, the simplest arrangement of an SMA actuator as a straight length of wire 10, however, remains somewhat awkward since the application of ohmic heating would require making electrical contacts to wire ends 12, 14 located at some distance from each other. End 12 is illustrated at anchor interface 16 and end 14 is illustrated at payload interface 18. Voltage potential is provided by wire 20 and ground is connected by wire 22.
Accordingly, folded geometries have been employed so as to allow both electrical contacts to be near one another. As shown in FIG. 2, SMA actuator wire 10′ is folded so that ends 12, 14 are near one another. An example of this folded geometry is given in FIG. 5 of U.S. Pat. No. 3,634,803. Another reason to prefer this geometry is that for a single fold, the two sides of the wire on each side of the fold account for a doubling of the available force compared to a single strand of the same cross section. Unfortunately, this benefit is accomplished by a doubling of the electrical resistance. When powered from a voltage source, the doubling results in halving the electrical current and the Joule heating power, while the heating time is significantly increased. If slower activation is not acceptable, the designer has the option of restoring the resistance of its initial value. One way to do this is by shortening the actuator by a factor of two. This time, the drawback is a reduction in stroke by the same factor.
As illustrated in FIG. 3, the other approach is to double the cross sectional area of the SMA wire 10″ by increasing the wire diameter by about 41%. This allows the designer to keep both the increased force and the previous stroke, while the activation time is actually improved, even with respect to the case depicted in FIG. 1. Unfortunately, this option also has disadvantages. The first is that—once again—the electrical contacts are inconveniently located at opposite ends 12, 14 of a long actuator. The second is that now the cooling time is significantly increased, by an amount even greater than the shortening of the contraction time.
What is needed is an actuator which can incorporate the force benefit of FIG. 2 and FIG. 3, the convenient electrical interface shown in FIG. 2, and the activation speed of the configuration shown in FIG. 3, while retaining the cooling speed of that shown in FIG. 2.